1. Field of the Invention
This invention relates to the field of automatic transmissions. More particularly, it deals with transmissions that produce a stepless, variable range of speed ratio by changing automatically the relative size of pulleys connected by an endless drive belt. The invention pertains to a system for controlling the speed ratio of such a transmission.
2. Description of the Prior Art
U.S. Pat. No. 4,767,384 discloses a system for controlling a stepless conical disc transmission having a primary input shaft driven by an engine or motor and a secondary output shaft driven by an endless drive belt. Conical disc assemblies, one located on the primary shaft and one on the secondary shaft, each includes a cylinder in which a piston, connected integrally to a conical disc of the respective disc assembly, moves within the respective cylinder to form a pressure chamber. The two pressure chambers are charged with high pressure to apply the necessary application pressure that develops between the conical discs and the drive belt on both the primary side and the secondary side.
To address the transmission ratio, two additional pressure chambers are located between the two conical disc members on the primary shaft. These two pressure chambers are separated mutually by a piston connected to one conical disc member so that the two conical discs are adjusted axially with respect to one another in accordance with the pressure within the two additional pressure chambers.
In the control system of the '384 patent, relatively low pressure produced by a pump to adjust the transmission ratio, i.e., to pressurize the two additional pressure chambers, is applied selectively to another pressure chamber. This low pressure can be substantially lower than the pressure used to maintain contact pressure between the drive belt and the conical pulley. The system of the '384 patent avoids the problem in continually variable transmissions of producing high pressure to maintain contact between the drive belt and the pulleys, for which a low displacement pump may be sufficient and high flow rate when adjusting the transmission ratio, and producing only moderately high pressure at other times. In the conventional method for supplying pressure through operation of one pump only, high volume flow at high pressure is supplied, although high flow rate is required only in extreme cases. In this way, a large percentage of the energy required to produce hydraulic power is converted continuously to useless heat. A further disadvantage of such systems is the need for three pressure chambers to produce contact pressure and to alter the transmission ratio. Furthermore, the need for three pressure chambers pressurized to different magnitudes is relatively expensive and complicated.
To adjust transmission ratio and to produce contact pressure dependent on torque, the system of the '384 patent requires that the pressures within the pressure cylinders be made unequal following an initial application of pressure of equal magnitude to the cylinders. This imbalance is required so that pressure in the cylinder with the lower pressure is monitored by an application pressure system, and the pressure in the other cylinder is increased by an additional increment.